CHAPTER XII ATMOSPHERIC CIRCULATION (Continued)

FORCED WINDS

Although, in greater or less measure, all winds are Interdependent, only a relatively small number obviously are generated and maintained by other, and coexisting, winds. Among these are eddy winds, Maloja winds, foehn or chinook winds, and, presumably, the winds of the tornado and waterspout.

Eddies.—Wherever the wind blows across a steep-sided hill or mountain, eddies are likely to be formed, especially on the lee side. In such cases, the direction of the surface wind is approximately opposite to that of the general or prevailing wind, with a calm between them.

Evidently we may consider a steady wind to consist of tubes of flow, in each of which, between fixed cross sections, the quantity of air and its energy remain constant. Furthermore, since thoroughly mixing air brings it to a constant potential temperature, or state of neutral equilibrium, it is clear that there is neither gain nor loss of gravitational energy in these tubes, whatever their inclination. Hence, the total energy is the volume energy, pv, in which p is the pressure and v the volume, and velocity energy \my? where m is the mass and u its velocity, at the given place along the tube. That is, along each tube, hence, any number of them

pv + -cJnu* = a constant or, dividing by v,

p + ~pu2 — a constant

Now, experiment shows that the product of the density of air, or other fluid, by the square of its velocity, is greatest where, or close to where, it flows past an obstruction. That is, at this place pu2 is a maximum, and therefore p is a minimum.

Hence the atmospheric pressure is reduced, near the crest, on the lee side, of a mountain across which a strong wind is blowing. This reduction of pressure causes air, already in neutral equilibrium, to flow gently up the lee side—a return or eddy wind—and there into the adjacent wind tube, thus maintaining the eddy.

For the practical purpose of the weather forecaster, wind eddies have but little significance, except in one important particular. He must

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